In order to develop a force actuator which is simple in design and easy to operate, designers have taken advantage of the physical properties of some metals. It is well known that when heat is applied to certain metals their shape and length will change significantly. One type of actuator which incorporates this type of metal is the bimetallic strip. These strips are constructed by laminating two pieces of metal together which have very different shape changing properties relative to temperature. Changes in temperature will cause one part of the strip to expand or contract at a different rate than the other which causes a deflection in the strip. A drawback of this type of actuator is that the total amount the strip will deflect is a small percentage of its overall length.
A class of metals that have particular properties relative to changes in temperature are shape memory alloys. When shape memory alloys are stretched or compressed at temperatures below their martensite temperature, they resume the size they had prior to the deformation if they are heated to above their austenite temperatures. One example of a prior art actuator which uses shape memory alloy is shown in FIG. 1. In this device, threaded cylinder 1 provides the output rotation for the actuator. Threaded onto the cylinder is translational nut 4. On either end of cylinder number 1 are stationary bearings 2. These bearings allow the output cylinder to rotate, but impede any translational movements. Connected between the stationary bearing 2 and the translational nut 4 are shape memory alloy springs 5 and 6. These springs have been trained such that they expand during the application of heat and contract when cooled. In operation, shape memory alloy spring 5 is heated by heat source 3. This spring expands and exerts a directional force on translational nut 4. The threads on this nut are designed such that when any translational force is applied, this force is transferred to the output cylinder 1 as rotation. Because the translational nut is not allowed to rotate, it moves in a rightward direction compressing the shape memory alloy spring 6. The number of rotations of cylinder 1 is limited by the thread spacing on the translational nut and body of the cylinder, the amount the shape memory alloy spring 5 will expand, as well as the original distance between the translational nut 4 and the stationary bearing 2.
The disadvantage of the rotational actuator shown in FIG. 1 is that if one desires to increase the number of rotations that the output cylinder may have in a particular direction, the size of the components must increase. For example, to increase the total number of rotations, the thread length on the output cylinder would have to increase. With a longer output cylinder, the size of the springs would also have to increase. Increases in the desired performance of the actuator significantly increase its overall size.
Therefore, it is desirable to have a constant force actuator which is simple in design and has the flexibility to change the amount of work the actuator performs without significantly changing the size of the actuator.